1. Field of the Invention
The present invention relates to an organic electro-luminescence display (OELD), and more particularly, to a driving method of an OELD that can improve the image quality.
2. Discussion of the Related Art
Until recently, display devices generally have employed cathode-ray tubes (CRTs). Presently, many efforts are being made to study and develop various types of flat panel displays, such as liquid crystal display devices (LCDs), plasma display panel (PDPs), field emission displays, and electro-luminescence displays (ELDs), as substitutes for CRTs. Of these flat panel displays, the PDP has advantage of large display size, but has disadvantage of low lightness and high power consumption. The LCD has advantage of thin profile and low power consumption, but has disadvantage of small display size. The OELD is a luminescent display and has advantage of fast response time, high lightness and wide viewing angle.
FIG. 1 is a cross-sectional view of an organic electro-luminescent diode of an OELD according to the related art.
In FIG. 1, the organic electro-luminescent diode includes an anode 2, a hole injection layer 3, an emitting layer 4, an electron injection layer 5 and a cathode 6 disposed in sequence on a substrate 1. The anode 2 and cathode 6 are supplied with driving voltages, and a hole in the hole injection layer 3 and an electron in the electron injection layer 5 move to the emitting layer 4 to emit light. Accordingly, the emitted light from the emitting layer 4 displays images.
In general, the OELD displays images with gray-levels by an area division driving method or a time division driving method. The area division driving method is a driving method that expresses gray-levels through multiple sub-pixels, which constitute one pixel and operate in accordance with multiple data signals corresponding to the multiple sub-pixels. Accordingly, the OELD driven by the area division driving method has a complex pixel structure. On the contrary, the time division driving method is a driving method that expresses gray-levels through multiple sub-frames, which constitute one frame interval. In the time division driving method, a pixel is on or off-state during each sub-frame. Accordingly, a gray-level is displayed by the summation of the on-state times of the sub-frames during one frame interval. Because the response time of the OELD is relatively fast compared with other flat panel displays, the time division driving method has been employed to drive the OELD efficiently.
FIG. 2 is a timing diagram for the time division driving method according to the related art to drive an OELD.
Table 1 shows an on-state time of each sub-frame to display a gray-level.
TABLE 1Sub-frameson-state time LTSF8SF7SF6SF5SF4SF3SF2SF1(weight value)1286432168421gray-level.........(data signal)..................1250111110112601111110127011111111281000000012910000001...........................
In FIG. 2 and Table 1, a data signal is an eight-bit binary code and has 256 (28) gray-level information. According to the time division driving method according to the related art, one frame interval F is divided into first to eighth sub-frames SF1 to SF8, and the first to the eight sub-frames correspond to the lowest to the highest bits of the eight-bit data signal, respectively. In other words, the first bit (the lowest bit) of the data signal corresponds to the first sub-frames SF1, and the second to the eighth bits of the data signal correspond to the second to the eighth sub-frames SF2 to SF8, respectively.
Each sub-frame SF has an on-state time LT and an off-state time UT. Because the pixels of the OLED are scanned along a vertical direction V-scan during each sub-frame SF, each on-state time LT follows the oblique line in FIG. 2 along the vertical direction V-scan. The on-state lime LT of each sub-frame SF corresponds to a weight value, which is the binary exponent of the binary code, of each bit of the data signal. Accordingly, the on-state time LT of each sub-frame SF is expressed in the form of a binary code, and the weight values of the first to the eighth on-state times LT1 to LT8 have the following relationship: LT1:LT2:LT3:LT4:LT5:LT6:LT7:LT8=20:21:22:23:24:25:26:27.
The pixel emits light when the corresponding bit of the data signal has a logic value “1”, and does not emit light when the corresponding bit of the data signal has a logic value “0” during each frame SF. Accordingly, the on-state time LT is the emission time of the pixel when the logic value is “1”. Thus, a gray-level can be displayed by the summation of the emission times during one frame interval F.
When gray-levels are displayed by the time division driving method according to the related art, all or some of the corresponding bits of the data signals, which display different gray-levels, may have different logic values.
For example, a first data signal is an eight-bit binary code of “01111111” for displaying the 127th gray-level, which is the (2n−1)th gray-level when n equals to 8. Further, a second data signal is an eight-bit binary code of “10000000” for displaying the 128th gray-level, which is the (2n)th gray-level when n equals to 8. While a first pixel supplied with the first data signal emits light during the first to the seventh sub-frames SF1 to SF7, a second pixel supplied with the second data signal emits light only during the eighth sub-frame SF8. Accordingly, the first pixel displaying the 127th gray-level and the second pixel displaying the 128th gray-level emit light alternatively. The percentage of the alternative emission time for the first pixel during the first to the eighth sub-frames SF1 to SF8 is 100% (127/127*100), and the percentage of the alternative emission time for the second pixel is also 100% (128/128*100).
In addition, a third data signal is an eight-bit binary code of “10011111” for displaying the 159th gray-level, when the first data signal is the eight-bit binary code of “01111111” for displaying the 127th gray-level. The first data signal and the third data signal have different logic values in the sixth, seventh and eighth bits. Accordingly, the first pixel displaying the 127th gray-level and a third pixel displaying the 159th gray-level emit light alternatively during the sixth to the eighth sub-frames SF6 to SF8. The percentage of the alternative emission time for the first pixel during the sixth to the eighth sub-frames SF6 to SF8 is 76% ((32+64+0)/127*100), and the percentage of the alternative emission time for the third pixel is 81% ((0+0+128)/159*100).
As shown by the above examples, all or some of the corresponding bits of the data signals, which display different gray-levels, may have different logic values. In addition, the alternative emission times occupies most of the emission times of the pixels, when the orders of the corresponding bits having different logic values are high.
Because the data signal is a multiple-bit binary code, and the on-state time of each sub-frame is expressed in the form of a binary code and is equal to the weight value (the binary exponent) in the related art, the on-state time increases with the binary exponent in accordance with the order of the bit. Accordingly, the pixels displaying different gray-levels emit light alternatively during most of the emission times, when the orders of the corresponding bits having different logic values are high. Thus, the OELD driven by the time division driving method according to the related art has problems in that a border flicker phenomenon occurs when a static image is displayed, and a dynamic false contour phenomenon occurs when a dynamic image is displayed.